A Chevy Tahoe I maintain for a friend had some weird brake issues - everytime the ABS cycles, the brakes will lock up and sometimes won't release. Yesterday I flushed the fluid - but the master cylinder diaphragm gasket was swollen and did recall a rag was wrapped on the reservoir before he put a proper cap on it. The fluid inside was black and gelled up to the consistency of a cold consomme.
I suspected contaminated fluid and I swapped out the master cylinder, flushed 1.5 quarts of fluid through all the wheels until it was mostly clear, did the "hillbilly" GM ABS bleed procedure by doing a few panic stops in an empty parking lot to cycle the modulator, I was expecting some air to be released and a soft/spongy pedal as a result - but my friend said there was no pedal change however he did say before we flushed the fluid when the ABS did cycle the wheels will lock up and he would be stopped in his tracks. We drove home and I pressure bled the system again, I did see a good bit of air escape. I used a total of 2.5 quarts of fluid. The truck does stop much better, the ABS system cycles as expected but there's some residual drag.
His suspicion was that his dad added power steering fluid to the system - I did find a bottle of Prestone-branded PS and brake fluid in the truck. The fluid level in the old MC was low. I did siphon out with a mityvac the old fluid and immediately swapped the MC to avoid pushing in old fluid. I know petroleum-based fluids in a glycol system are a no-no, but does plain neglect of a brake system, meaning no brake fluid flushes can also cause brake fluid to congeal and gel?
I suspected contaminated fluid and I swapped out the master cylinder, flushed 1.5 quarts of fluid through all the wheels until it was mostly clear, did the "hillbilly" GM ABS bleed procedure by doing a few panic stops in an empty parking lot to cycle the modulator, I was expecting some air to be released and a soft/spongy pedal as a result - but my friend said there was no pedal change however he did say before we flushed the fluid when the ABS did cycle the wheels will lock up and he would be stopped in his tracks. We drove home and I pressure bled the system again, I did see a good bit of air escape. I used a total of 2.5 quarts of fluid. The truck does stop much better, the ABS system cycles as expected but there's some residual drag.
His suspicion was that his dad added power steering fluid to the system - I did find a bottle of Prestone-branded PS and brake fluid in the truck. The fluid level in the old MC was low. I did siphon out with a mityvac the old fluid and immediately swapped the MC to avoid pushing in old fluid. I know petroleum-based fluids in a glycol system are a no-no, but does plain neglect of a brake system, meaning no brake fluid flushes can also cause brake fluid to congeal and gel?